1. Statement of the Technical Field
The inventive arrangements relate to directional networks that can cover terrestrial and airborne nodes over hundreds of nautical miles, and more particularly to long range TDMA time slot scheduling in peer-to-peer directional networks.
2. Description of the Related Art
Peer-to-peer mobile ad hoc networks (MANET) using directional antennas are known in the art. These MANET networks can use a variety of communications formats including Time Division Multiplexed Access (TDMA) arrangements. Further, they are characterized by independence from fixed infrastructures and by peer-to-peer distributed control mechanisms and protocols. In TDMA type MANET networks, each node can communicate during a specified time period or time slot. In order to coordinate such communications, each node can include a clock which is synchronized with a highly stable time reference. For example, this stable time reference can be derived from a satellite-based GPS signal. It should be noted that TDMA cellular systems often rely on synchronization to base stations to establish system timing and to compensate for propagation delay differences. Such TDMA cellular systems include, but are not limited to, global systems for mobile communications (GSM), Integrated Digital Enhanced Network (iDENs) systems, 802.16 based broadband wireless access network systems, and TDMA satellite communications (SATCOM) systems.
Still, each TDMA node can be expected to have a significant total time base uncertainty. Also, propagation delay between nodes can create additional timing uncertainties that change in real time as the mobile nodes move relative to one another. In order to manage these timing uncertainties, prior art systems use a base station infrastructure to create a hub and spoke typology. These base stations are usually, but not necessarily, fixed in location. The mobile nodes operate as “spokes” or “clients” to the basestation hubs and synchronize their transmissions to those base stations.
A MANET is an infrastructureless network that uses peer-to-peer control mechanisms and therefore does not have base stations to provide a central timing reference. Therefore, there is a need in a TDMA MANET for a new time synchronization method that can compensate for variable propagation delay and other propagation uncertainty in the absence of a central infrastructure.
In a TDMA MANTET, a transmitted RF burst is can be transmitted within a time slot as close to the beginning of the time slot as possible. The time slot length can be selected to exceed the maximum RF burst length by an amount of time referred to as the “guard time.” This guard time is used to accommodate any timing uncertainties at either the transmitter or receiver, and the maximum propagation time delay. If the range delay between the transmit node and the receive node exceeds the allocation in the guard time, a maximum length RF burst will spill over into the following time slot at the receiver. This spillover can cause interference if such following time slot is in use for communications as between other nodes.
Conventional MANET networks using TDMA generally choose a guard time which is based (at least in part) on the maximum permitted range between nodes. In this regard, networks which are designed to accommodate larger distances between nodes can require larger guard times. These larger guard times are necessary to accommodate potentially longer propagation delays between distant nodes. This approach can provide acceptable results for a limited network range. However, for networks operating over hundreds of nautical miles, the guard time can become relatively lengthy in duration. Such lengthy guard periods restrict the available time for other nodes to communicate with each other, leading to inefficiencies in the overall network. For example, a system which has 88% efficiency with an operating range of 11 nautical miles can have an efficiency of only 16% if the operating range is extended to 440 nautical miles.
In order to address this problem, several existing TDMA wireless systems have used range adjusted timing systems. However, such systems have generally only been applied in hub-spoke type networks. The problem becomes more considerably more complex in the case of ad-hoc peer-to-peer networks. Accordingly, a new approach is needed for ad-hoc peer-to-peer communications networks to facilitate TDMA communications which are capable of providing efficient communications over larger geographic areas.